The present invention relates to a medium for optically recording information.
The optical type information recording medium such as an optical disk can record and reproduce a large amount of data from a single disk at high density. Therefore, recently, the optical disk has been often used as a recording medium for video or audio information, etc.
In an apparatus using the optical disk, as is well-known, for example, a laser beam is reduced to about 1 .mu.m in diameter and has 1.5 to 2 .mu.m in pitch so that a recording track having a concentric or spiral shape is formed to record a signal and the reproduction is performed by using the laser beam.
The conventional technology relating to cross-sectional shapes of a guide groove for tracking and a pre-format pit for recording address information, etc. is disclosed in Japanese Laid-Open Patent Nos. 58-102347, 59-40342 and 59-168945 and Japanese Published Patent No. 63-19939, etc., for example. However, in the technology disclosed in such patent publications, the shapes of the guide groove, etc. are different from those in a land recording system and the depth of the pit is shallow such as about (1/8).lambda. or 0.25.lambda./n to 0.29.lambda./n nanometers where n is a refractive index of a substrate and .lambda. is a wavelength of the laser beam. Further, the shapes of the pit, etc. are represented by angles, gauss distribution, etc., and it is thereby difficult to specify the real shapes thereof.
Therefore, with respect to the optical disk in the land recording system manufactured on the basis of the conventional technology mentioned above, there are some problems such as a disturbance, the reduction in reflectance of the optical disk with the passage of time, a shift in optical axis of the laser beam, the occurrence of the inclination of a surface of the optical disk, etc. Therefore, the tracking error tends to be caused often. In a severe case, the amplitude of a tracking error siganl is reduced to that less than 50% in comparison with a predetermined amplitude so that it is impossible to normally perform a tracking servo control.